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There is no universal answers for stuff like this, see it as an opportunity to educate yourself. Ask yourself why you are doing a water fast, when you know why ask yourself if taking a fiber supplement will prevent you from getting the result you want out of your fast. If not, go ahead. If it does then maybe don’t.
I agree that the purpose of your fast is relevant to answering this question. Otherwise, I’m not sure what you mean by “ok”? But according to Natural Hygienists, fasting is one of the quicker ways to allow the body to heal because digesting food is one of the most energy-intensive things our bodies do. By not eating (paired with rest), fasting allows our bodies to redirect that energy, putting it to use healing.
If you eat/take anything the body has to digest (fiber supplements, coffee, tea, etc.), you won’t technically be fasting as your body will need to kick into digestion mode, even if only a little. While not technically a fast, this would, however, still greatly reduce the amount of energy expenditure for digestion and would still increase healing, but the full benefits of fasting are achieved with water only–when the body doesn’t have to start the digesting process at all. But if quick-as-possible healing isn’t your main goal, then this isn’t necessarily relevant. If your goal is just to lose weight, fasting isn’t a great way to go about that due to the temporary/unsustainable nature of it, which can lead to yo-yoing. If your goal is something else entirely, then the answer might be totally different. So again your intention here matters.
I’m not a doctor or expert, but I have done 2 fasts of 15 days, 1 of 30, and many of 1-5 days (for the purposes of healing Ulcerative Colitis), with a fair bit of study in Natural Hygiene (which, to be honest, is not a terribly common or agreed-with approach, but in my experience I’ve found it to have merit). Take the above with a grain of salt, and it is highly recommended that any water fast over 3 days should be done under proper supervision.